Surgical Theatre Ventilating Devices and Methods

ABSTRACT

Provided are methods for ventilating a surgical theater using temperature-regulated laminar air flow. Velocity of a downward directed laminar clean air flow is determined by an air-temperature difference between the supply air and room air temperature at the level of the operating table. Room air temperature at the level of the operating table is measured and clean supply air temperature controlled in relation to this measurement. In order to maintain a constant downward directed laminar clean air flow velocity, a constant difference in temperature is maintained between room air temperature at the level of the operating table and the lower temperature of the supply air. In preferred embodiments, this constant temperature difference provides a downward directed air flow velocity of at least 0.25 m/s and is maintained in part by minimizing fluctuations in ambient air-temperature through use of air supply units supplying heated or cooled air outside the clean air zone. Also provided are ventilating devices which create a uniform and stable downward laminar air flow that forms a clean air zone surrounding the operating table workplace region. Preferred embodiments comprise a number of air supply units arranged in a closed pattern, e.g, in a circle, with air stop and guide units situated between air supply units such that a widely spread uniform and stable, downward, combined, laminar air flow is created.

FIELD OF THE INVENTION

The present invention relates in general to devices and methods forproviding a zone of clean air in the operating table workplace region ofa surgical theater, and in particular, to methods and devices thatuitilize temperature controlled laminar air flow.

BACKGROUND

Surgical site infections (SSIs) are the second most common cause ofhospital acquired infections. 1.5% to 20% of surgical operations leadsto a Surgical Site Infection (SSI), depending on the type of surgicalprocedure and the wound classification.

Patients who develop SSIs suffer significant debilitation and increasedrisk. Patients with SSIs have up to 60% increased likelihood ofhospitalization in an intensive care unit. Patients with SSIs have 5times greater likelihood of readmission to the hospital and 2 timesgreater risk of death than patients without SSIs.

Societal costs for SSI's are substantial. European studies shows thatthe average extended length of stay for an infected patient is 9.8 days.The cost per SSI patient is between £1,862 to £4,047 in direct costs inhospital costs alone. From 30 million surgical procedures a year theresulting numbers of SSIs amounts to 0.45 to 6 million, giving rise to atotal SSI cost in Europe of somewhere between £1.47 to £19.1billion/year. Studies from USA show similar figures with an averageextended length of stay for an infected patient of somewhere between 7to 10 days. The cost per SSI patient between $8,200 to $42,000 includingindirect costs. With approximately 0.5 million SSI cases per year, totalSSI cost in the USA is in the range between $1 to $10 billion/year.

The primary contributing cause to development of surgical site infection(SSI) is generally acknowledged to be bacterial contamination of theoperating room air either directly contaminating the patient's wound orindirectly contaminating sterile surgical equipment.

It is also generally accepted that the origin of this bacterialcontamination of operating room air is predominantly contaminated skinscales shed from surgical team members.

Pre operative actions have proved effective in reducing risk of SSIs,including: Antimicrobial prophylaxis, preparation of the patient,hand/forearm antisepsis for surgical team members, and management ofinfected or colonized surgical personnel. Postoperative incision careand postoperative surveillance have also proved effective in reducingrisk of SSIs.

Other promising measures for preventing SSIs focus on activities in theoperating theater, during the course of the operation. Cleaning anddisinfection of environmental surfaces, microbiologic sampling,sterilization of surgical instruments, surgical attire and drapes, andimproved asepsis and surgical techniques have all been reported. Ofparticular interest, improved clean air ventilation in the operatingtheater has been shown to reduce risk of SSIs. Charnley et al. reportsthat vertical laminar airflow systems and exhaust-ventilated clothingcan decrease the risk of attaining a SSI from 9% to 1%. Lidwell et al.has, comparing the effects of laminar airflow systems and anti-microbialprophylaxis in a study of 8,000 total hip and knee replacements,measured a decrease in SSI rate from 3.4% to 1.6% simply from use oflaminar airflow systems. It is now generally understood that verticalLaminar Air Flow (LAF) systems in surgical theaters provide the mosteffective techniques for reducing the numbers of bacteria-carryingparticles within the operative area.

However, some problems with vertical laminar air flow systems yetremain. The main source/s of bacteria-carrying particles (skin flakes)are the personnel within the surgical theatre. The most physicallyactive operative personnel operate within the actual boundaries of thelaminar air flow.

Skin flakes shed from operative personnel/human bodies must be preventedfrom reaching the patient's exposed wound. In order to accomplish this,the descending laminar air flow should brake, and immediately bringdownwards, lighter/warm convection air flow generated from the warmhuman bodies of operative personnel and carrying potentially infectiousskin flakes. These particles can then be evacuated at floor level.

In order to be effective in braking human body convection flows, thevelocity of the downward directed laminar air flow needs to be at leastabout 0.25 m/s as measured at the levels of the patient's exposed wound.This downward velocity needs to be maintained constant during the entireoperation. Higher velocities above about 0.25 m/s cause familiarproblems of draft and dehydration for operative personnel and, further,give rise to turbulent air flows which compromise the advantages of alaminar flow system.

The velocity of a free-flowing vertical laminar air stream with alimited cross section is either enforced or repressed depending on thetemperature difference between the flowing air and the ambientstill-standing volume of air. Cold air has a higher density than warmerair and vice versa. A free-flowing vertical laminar air stream which isrelatively colder than the ambient air volume will descend/fall as longas this difference in density (temperature) is maintained. In order toestablish a downward directed (vertical) laminar air stream flowing(falling) through an air volume with an equal or lower temperature, aset up is required with aligned supply- and exhaust air devices havingrelatively tight distances between. In surgical theatres, this becomesexpensive, space demanding and limiting for surgical procedures and foroperative personnel.

More advanced LAF systems cool and control the supply air temperature bykeeping it constant to a set temperature, which can be adjustedaccording to the demands of the operative personnel and type of surgicalprocedure. However, these systems are intended to control thetemperature for the operative personnel working beneath the ceilingmounted LAF air delivery devices. They do not adjust the supply airtemperature according to varying temperature within the theatre. Inactual practice, room temperature fluctuations can occur due to varyingheat loads including heat from operative personnel, surgical lights,other electric equipment, surrounding surfaces and in some casessunlight. Further, these LAF devices of the prior art utilize forcedblowing as the driving force for controlling the downward directed airvelocity. This forced blowing generally entails a high initial airvelocity of at least double the desired velocity at the operating table.This in turn results in disturbing effects, e.g. turbulence, arisingfrom, for example, operating lighting or other equipment situatedbetween the ventilating device and the workplace region. This turbulenceis associated with in-mixing of contaminated ambient air into the cleanair flow. The high air velocity also creates strong secondary air flowsoutside the workplace region which keep bacteria-bearing and otherparticles suspended, increasing the risk of contamination of theworkplace region. High air flow velocity also subjects personnel todraughts and high noise levels. Further, room temperature fluctuationsmay result in fluctuations of the actual downward directed velocityduring and between surgery.

The problems associated with forced-blowing systems can be avoidedthrough use of temperature-controlled laminar air flow. The principle oftemperature controlled laminar air flow (TLA) is that a laminar flow isinduced by an air-temperature difference between supply air and ambientair at the level of the operating table. A laminar flow of filtered,colder air, having a higher density than ambient air descends slowly,enveloping the operating table workplace region. Because the supply airflow is substantially laminar, and in-mixing with ambient air isminimized, the air-temperature difference is maintained throughout thepath of descent. Only minimal impulse is imparted to the supply airstream, sufficient to overcome resistance at the outlet nozzle.

Here we describe improved air supply devices as well as methods fortemperature-controlled laminar air flow ventilation, providing anenforced temperature and velocity controlled air stream enveloping theoperative area and outside the operative area an equally controlledenvironment covering the entire theatre.

SUMMARY

Some embodiments of the invention provide methods for ventilating asurgical theater using temperature-regulated laminar air flow. Velocityof a downward directed laminar clean air flow is determined by anair-temperature difference between the supply air and room airtemperature at the level of the operating table. Room air temperature atthe level of the operating table is measured and clean supply airtemperature controlled in relation to this measurement. In order tomaintain a constant downward directed laminar clean air flow velocity, aconstant difference in temperature is maintained between room airtemperature at the level of the operating table and the lowertemperature of the supply air. In preferred embodiments, this constanttemperature difference provides a downward directed air flow velocity ofat least 0.25 m/s and is maintained in part by minimizing fluctuationsin ambient air-temperature through use of air supply units supplyingheated or cooled air outside the clean air zone. Also provided areventilating devices which create a uniform and stable downward laminarair flow that forms a clean air zone surrounding the operating tableworkplace region. Preferred embodiments comprise a number of air supplyunits arranged in a closed pattern, e.g, in a circle, with air stop andguide units situated between air supply units such that a widely spreaduniform and stable, downward, combined, laminar air flow is created.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a ventilating device according to theinvention and the air flows generated by it.

FIG. 2 is a somewhat enlarged side view of a container with air supplyunits, and with air stop and guide units disposed between the air supplyunits, for the ventilating device shown in FIG. 1.

FIG. 3 is a cross-sectional plan view of the container with the airsupply units and the air stop and guide units according to FIG. 2.

FIG. 4 is an enlarged side view of part of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In some embodiments, the invention provides a method for ventilating asurgical theater comprising

-   -   Discharging a purified air stream through an air supply device,        situated above the operating table workplace area, as a        substantially laminar descending air flow with velocity        determined by the difference in air-temperature between the        supplied air and the ambient air at the level of the operating        table

wherein a constant difference in air-temperature between the suppliedair and the ambient air at the level of the operating table ismaintained in part by use of air supply units providing heated or cooledsupply air outside the operative area in order to minimize fluctuationsin ambient air-temperature.

FIG. 1 shows one preferred embodiment of a ventilating device suitablefor practice of methods of the invention. The device shown in FIG. 1 isintended to create a zone 1 of clean air between the ventilating deviceand a workplace region, here the operating region 2 in a surgicaltheater. The ventilating device comprises air supply units 3 which maybe of a conventional type and are adapted to generating laminar airflows intended to constitute said clean air zone 1.

It is advantageous to achieve a total air flow with a large spread whichtherefore serves a large region within which personnel have freedom ofmovement for their work. In some preferred embodiments, the ventilatingdevice according to the invention comprises at least three air supplyunits 3 disposed in a closed trilateral pattern of three air supplyunits. The result is that the clean air zone 1 has below the air supplyunits 3 an extent which in cross-section substantially corresponds tothe surface delineated by said closed pattern of air supply units andthe surface situated within that pattern, i.e. substantially the extentindicated by FIG. 1. In other embodiment, a single large air supply unitmay be used, for example, a large ring-shaped unit.

To prevent or hinder air surrounding the clean air zone 1 and containingbacteria bearing and other pollutant particles from being drawn inbetween the air supply units and into the clean air zone by the negativepressure and consequent suction force generated in the clean air zone bythe air flows of the mutually adjacent air supply units 3, somepreferred embodiments comprise in addition a corresponding number of,i.e. at least three, air stop and guide units 4 disposed between therespective pairs of mutually adjacent air supply units.

As well as being trilateral or circular as indicated above, the closedpattern of air supply units 3 may also be, for example, elliptical,square, rectangular or have five, six or more sides or a combination ofdifferent shapes. In such cases, the air stop and guide units aresuitably disposed in corresponding patterns in the spaces delineatedbetween mutually adjacent air supply units 3. Each air stop and guideunit 4 will with advantage also fill the whole space between twomutually adjacent air supply units 3.

The number of air supply units 3 and the number of air stop and guideunits 4 disposed between them each amount preferably to between 3 and15, depending on the desired extent of the region to be served by theventilating device. In the preferred version depicted in the drawings,the number of air supply units 3 and air stop and guide units 4 is eight(8) each.

The air supply units 3 and the air stop and guide units 4 disposedbetween them in the version depicted are mounted on a container 5. Thecontainer 5 is fitted permanently in the ceiling of the room in whichthe workplace region is situated, i.e. here in the ceiling 6 of theoperating room 7 in which the operating region 2 defining orconstituting the operating table 8 is situated.

The container 5 comprises with advantage, or is connected via an airduct 9 to, at least one air intake for taking air in from the room 7and/or from at least one location outside said room. Thus, for example,some of the air drawn out of the room 7 via air extracts 10 at or nearthe floor 11 of the room may be led back to the air supply units 3 inthe ventilating device. Air may also be brought from air intakes (notdepicted) in or near the ceiling 6 of the room 7.

The container 5 comprises with advantage, or is likewise connected viapreferably the same air duct 9 to, a fan device (not depicted) forsupplying air and causing it to flow through the air supply units 3.

Correspondingly, the container 5 comprises, or is connected preferablyvia same air duct 9 to, an air treatment device for generating clean airfor the clean air zone 1. The air treatment device comprises in a simpleversion at least one filter device (not depicted) for filtering the airto the air supply units 3 so that the air will be clean and canconstitute said clean air zone 1, and also a device (not depicted) forcooling of air from the filter device to a lower temperature than thetemperature of the air in the room 7, so that clean air intended toconstitute the clean air zone will be at such a lower temperature, e.g.1-2° C. lower, than air surrounding the clean air zone that clean air inthe clean air zone sinks slowly downwards towards the workplace region,here the operating table workplace region 2. The higher density of thecooler air is thus used for controlling the downward velocity. In someembodiments, it may be advantageous to maintain a low velocity, that is,a small air temperature difference between ambient and supply air, forexample between 0.3 and 1° C., or between 0.5 and 1° C. Filtered air istypically forced out of the air supply unit with only enough dynamicpressure sufficient to overcome resistance in the air supply nozzle andthe rest of the device. This initial velocity is quickly counteracted bythe static pressure of ambient air, such that continued downward descentof supply air a few centimeters away from the supply unit is determinedby the air temperature difference. The air temperature difference needonly be sufficient to provide the velocity required at the workplaceregion for maintaining a clean air zone. Where the supply air flow issubstantially laminar, and in-mixing with ambient air is avoided, theair-temperature difference is maintained throughout the path of descent.Fewer disturbing effects, turbulence, and secondary air flows outsidethe workplace region are thereby generated, resulting in less risk ofcontamination of the workplace region. Low air velocity results in smallair flow with high efficiency and, for personnel, a draught-free andquiet work environment.

The level of the preferably constant lower temperature of the air in theclean air zone 1 relative to surrounding air in the room 7 is withadvantage maintained by a regulating device (not depicted) which formspart of the ventilating device and which therefore regulates thetemperature of the clean air in the clean air zone in order to regulatethe velocity of the clean air in the clean air zone. To this end, theregulating device is controlled by air temperature sensors of a suitabletype. In preferred embodiments, one sensor is situated in the supplyclean air (8) for the clean air zone of the operating room while asecond and possibly a third sensor is situated outside the clean airflow at the level of the operating table (19). Including two sensors formeasuring the room temperature at the level of the operating tableallows for a mean value to be calculated reducing the risk of error. Italso allows for an alarm to be given if the difference between thesensors is too high. The sensors are preferably placed far aside i.e. onopposite walls each side of the operating table.

The air supply units 3 and the air stop and guide units 4 disposedbetween them are preferably fitted at or in the vicinity of the outerperiphery of the container 5 if the shape of the container is differentfrom the closed pattern which said air supply units and air stop andguide units form.

As depicted in FIG. 1, a lighting device with one or more lamps 12suspended in arms 13 may be situated close to the container 5.

In the depicted preferred version, the container 5 takes the form of acontainer 14 with the air supply units 3 and the air stop and guideunits 4 disposed between them fitted on the underside of the container.The container 14 is here circular with a diameter of about 1 to 4 m. Theclosed circular pattern of air supply units 3 and air stop and guideunits 4 runs along and close to the outer periphery of the container 14.

The respective air supply units 3 in the ventilating device may be ofthe type described in, for example, PCT/SE2004/001182, which is herebyincorporated by reference herein in entirety. Thus the respective airsupply units 3 as seen from the side may preferably be of at leastpartly hemispherical or substantially hemispherical shape, resulting ina distinct clean air zone with a distinctly limited extent from each airsupply unit. The respective air supply units 3 also preferably present asubstantially circular cross-section. Each air supply unit 3 has a body15 made of foam plastic or similar porous material or fabric adapted togenerating laminar air flows, thereby minimizing the risk of airsurrounding the clean air zone 1 entering the clean air zone. The body15 may comprise an inner element and an outer element, the inner elementimparting to air flowing through a greater pressure drop than the outerelement. The inner element may be made of foam plastic or other porousmaterial or fabric, while the outer element takes the form of, forexample, tubular throughflow ducts. The length of these throughflowducts is with advantage 4-10 times greater than their width, to ensurethat the turbulence in at least an outer portion of the clean air zone 1will be as little as possible. Other suitable types of air supply unitswith desired suitable functions may nevertheless be used in theventilating device according to the present invention.

The form of the respective air stop and guide units 4 will beappropriate to the desired function. In the version depicted, each airstop and guide unit 4 comprises accordingly at least one air stopsurface 16 which faces away from the clean air zone 1 and prevents orhinders air surrounding the clean air zone from being drawn in betweenadjoining air supply units 3 and into the clean air zone. Each air stopand guide unit 4 also comprises at least two first air guide surfaces 17which run from the air stop surface 16 in between adjoining air supplyunits 3, converge towards one another and guide away from one anotherand out from the centre of the clean air zone 1 parts of the respectiveair flows directed towards one another from adjoining air supply units.Each air stop and guide unit 4 also comprises at least two second airguide surfaces 18 which face inwards towards the centre of the clean airzone 1 and towards said first air guide surfaces 17, converge towardsone another and guide away from one another and inwards towards thecentre of the clean air zone parts of the air flows directed towards oneanother from adjoining air supply units 3. This preferred version of theair stop and guide units 4 achieves the least possible turbulencebetween the air flows meeting between the air supply units 3 andprevents bacteria-bearing and other pollutant particles from being drawninto the clean air zone 1.

As the respective air supply units 3 in the preferred version depictedare substantially circular in shape, the respective air stop and guideunits 4, especially their first air guide surfaces 17, run here along atleast about 90″ of the periphery of adjoining air supply units.

The air stop surface 16 on the air stop and guide units 4 has withadvantage a configuration which in at least a cross-sectional planethrough said surface and through the air supply units 3 coincides withthe configuration of a line which links the outermost portions of theair supply units as seen from the clean air zone 1. As shown in FIG. 3,in the preferred version depicted with the air supply units 3 disposedin a circle, the air stop surface 16 has accordingly a curvature whichin said cross-sectional plane coincides with the curvature of a circularline which runs through the radially outermost portions of the airsupply units. The air stop surface 16 is also preferably of such alength that it runs from the vicinity of the outermost portions of oneof the two mutually adjacent air supply units 3 between which therespective air stop and guide unit 4 is disposed, to the vicinity of theoutermost portions of the other of the two air supply units. Thiscontributes to optimum filling of the space between each pair ofmutually adjacent air supply units 3.

As shown in FIG. 3, in the preferred version depicted with the airsupply units 3 disposed in a circle, the first air guide surfaces 17 onthe respective air stop and guide unit 4 as seen in a cross-sectionalplane converge towards one another preferably in a manner correspondingto the cross-sectional shape of adjoining air supply units 3, i.e. saidsurfaces run towards one another inwards towards the centre of the cleanair zone 1 and have accordingly the same configuration as adjoining airsupply units so that the distance between the first air guide surfacesand the air supply units is constant.

The first air guide surfaces 17 as seen in a longitudinal sectionalplane also converge towards one another, i.e. said surfaces run towardsone another downwards to the workplace region 2 in the clean air zone 1(see FIGS. 2 and 4).

Finally, the second air guide surfaces 18 run, as above, towards thefirst air guide surfaces 17 outwards from the centre of the clean airzone I and downwards towards the workplace region in the clean air zone(see FIGS. 2-4). They also run towards one another downwards towardssaid workplace region (see FIGS. 2 and 4).

With the object of also controlling the level of bacteria-bearing andother pollutant particles outside the clean air zone the workplaceregion 2 and preventing or hindering any occurrence of “whirlpools” ofsecondary air flows holding such particles in suspension, it isadvantageous if air is also supplied in a controlled manner outside theclean air zone. To this end, according to the invention, at least onefurther air supply unit 3, preferably providing a flow of purified air,is disposed in the room 7 to supply air to the room. This air maintainswith advantage a temperature exceeding the temperature of the air in theclean air zone 1, thereby compensating in particular for the coolingeffect caused by the clean air zone 1. In the preferred versiondepicted, a plurality of further air supply units 3 are disposed allround the first-mentioned air supply units 3 and said air stop and guideunits 4 (on the container 5) in the room 7 to supply the room round theclean air zone with somewhat warmer air than the air in the clean airzone 1. Said further air supply units 3 have their own, or are suitablyconnected at least to the aforesaid, fan and filter devices.

Accordingly, a method for temperature-regulated laminar air flowventilation of a surgical theater is also provided. The room airtemperature at the level of the operating table is measured by a sensor19 and the supply air temperature controlled in relation to thismeasurement, thereby controlling the corresponding velocity of thedownward directed laminar air flow at the desired level. In order tomaintain a constant downward directed laminar air flow velocity, aconstant difference in temperature is maintained between room airtemperature at the level of the operating table and the lowertemperature of the supply air. In preferred embodiments, this constanttemperature difference provides a downward directed air flow velocity ofat least 0.25 m/s and is maintained by air supply units supplying heatedor cooled air outside the operative area. As used herein, the term“constant” as applied to temperature refers to a level that is within+/−0.5 degree C. The term “constant” as applied to temperaturedifference refers to a level that is maintained within +/−0.5 degrees C.The term “constant” as applied to room temperature refers to a levelthat is maintained within +/−1 degree C. The term “constant” as appliedto air flow velocity refers to a level that is maintained within +/−40%.In preferred embodiments additional clean air supply devices maintainconstant room temperature by introducing warmed or cooled air in acontrolled manner. For example, using air supply devices described inPCT/SE2004/001182, 60% of the supply air (providing supply air at afixed lower air temperature than ambient room air temperature to securethe correct downward directed velocity) can be supplied usingventilation devices of the invention. The additional 40% of supply aircan be supplied by external air supply devices (providing supply air ata higher temperature to maintain a required room temperature. 100% ofthe supply air can be evacuated at floor level. In this manner theentire room will be served by steady downward directed laminar airflowsof different velocities. The room temperature can be adjusted to anylevel required by operative personnel or by a surgical procedure withoutaffecting the temperature difference and thereby the downward directedvelocity at the point of surgery.

Ventilating devices according to the invention may further comprise aregulating device (not depicted) for regulating the temperature of theair which is supplied to the room 7 and caused to surround the clean airzone 1, and/or for regulating the velocity of the air which is suppliedto the room and is caused to surround the clean air zone. Thetemperature of the whole room 7 can thereby be regulated. The regulatingdevice is controlled by temperature sensors situated in the room 7outside the clean air zone 1.

It will be obvious to one skilled in the art that ventilating devicesaccording to the invention can be modified and altered within the scopeof the claims set out below without departing from the idea and objectof the invention. Thus, for example, said fan, filter and coolingdevices may be configured and disposed in any manner appropriate to thepurpose, as also may said regulating devices. The number, type and shapeof the air supply units and of the air stop and guide units may varybeyond what is indicated above, as also may how they are positionedrelative to one another and how they are positioned on the container forthe ventilating device. The shape of the container may also vary beyondwhat is indicated above and may also, as previously indicated, follow ornot follow the closed pattern constituted by the air supply units andthe air stop and guide units.

1-6. (canceled)
 7. A method for ventilating a surgical theatercomprising: discharging a purified air flow through an air supplydevice, situated above an operating table workplace area, as asubstantially laminar descending air flow with a velocity determined bya difference in air-temperature between the supplied air and the ambientair at the level of the operating table such that the air flow is coolerthan the ambient air, wherein the velocity of the downward directed flowis maintained at at least 0.25 m/s as measured at the level of anexposed wound of a patient on the operating table, whereby humanconvection currents are braked inside the operating table workplacearea, and wherein the difference in air-temperature between the suppliedair and the ambient air at the level of the operating table ismaintained in part by use of air supply units providing heated or cooledair outside the clean air zone surrounding the operating table workplacearea.
 8. The method of claim 7 wherein the air-temperature differencebetween clean supply air and room air temperature at the level of theoperating table is maintained in the range of about 0.3 to 1° C.
 9. Themethod of claim 7 wherein the air-temperature difference between cleansupply air and room air temperature at the level of the operating tableis maintained in the range of about 0.5 to 2° C.
 10. A method forventilating a surgical theater comprising: discharging a purified airflow through an air supply device, situated above the operating tableworkplace area, as a substantially laminar descending air flow with avelocity determined by the difference in air-temperature between thesupplied air and the ambient air at the level of the operating tablesuch that the air flow is cooler than the ambient air, wherein aconstant difference in air-temperature between the supplied air and theambient air at the level of the operating table is maintained in part byuse of air supply units providing heated or cooled air outside the cleanair zone surrounding the operating table workplace area and bodyconvection currents are braked, and wherein the method is practicedusing a ventilating device for providing a zone of clean air between theventilating device and a workplace region in a room, which ventilatingdevice comprises air supply units adapted to generate laminar air flowsintended to constitute said clean air zone, wherein the ventilatingdevice comprises at least three air supply units disposed in a closedpattern so that the extent in cross-section of the clean air zone belowthe air supply units substantially corresponds to the surface delineatedby said closed pattern of air supply units and the surface situatedwithin that pattern, and a corresponding number of air stop and guideunits disposed between, and substantially filling the space between,each pair of mutually adjacent air supply units, each air stop and guideunit having at least one air stop surface which faces outwards away fromthe clean air zone and prevents or hinders air surrounding the clean airzone from being drawn in between adjoining air supply units and into theclean air zone, at least two first air guide surfaces which run from theair stop surface in between adjoining air supply units, converge towardsone another and guide parts of the air flows from adjoining air supplyunits that are directed towards one another away from one another andoutwards from the center of the clean air zone, and at least two secondair guide surfaces which face inwards towards the center of the cleanair zone and converge towards said first air guide surfaces and towardsone another and guide other parts of the air flows from adjoining airsupply units that are directed towards one another away from one anotherand inwards towards the center of the clean air zone.
 11. A ventilatingdevice for providing a zone of clean air between the ventilating deviceand a workplace region in a room, which ventilating device comprises airsupply units adapted to generate laminar air flows intended toconstitute said clean air zone, wherein the ventilating device comprisesat least three air supply units disposed in a closed pattern so that theextent in cross-section of the clean air zone below the air supply unitssubstantially corresponds to the surface delineated by said closedpattern of air supply units and the surface situated within thatpattern, and a corresponding number of air stop and guide units disposedbetween, and substantially filling the space between, each pair ofmutually adjacent air supply units, each air stop and guide unit havingat least one air stop surface which faces outwards away from the cleanair zone and prevents or hinders air surrounding the clean air zone frombeing drawn in between adjoining air supply units and into the clean airzone, at least two first air guide surfaces which run from the air stopsurface in between adjoining air supply units, converge towards oneanother and guide parts of the air flows from adjoining air supply unitsthat are directed towards one another away from one another and outwardsfrom the center of the clean air zone, and at least two second air guidesurfaces which face inwards towards the center of the clean air zone andconverge towards said first air guide surfaces and towards one anotherand guide other parts of the air flows from adjoining air supply unitsthat are directed towards one another away from one another and inwardstowards the center of the clean air zone.